The present invention relates to a liquid crystal display (LCD) having a liquid crystal display panel (LCD panel) of a passive matrix display type, and more particularly, relates to a liquid crystal display having little display irregularity in which a plurality of scanning electrodes (rows) of a liquid crystal display panel are simultaneously driven.
As a method of driving a liquid crystal display panel of a passive matrix display type, a voltage averaging method described in xe2x80x9cLiquid crystal display handbookxe2x80x9d pp. 395-399, ISBN 4-526-02590-9 C 3054, published on Sep. 29, 1989, in Japan (in Japanese), is widely employed. According to the method, scanning electrodes corresponding to a row in the liquid crystal display panel are sequentially selected every one scanning period, a selective scanning voltage is applied, and all of scanning electrodes are scanned during a period of one frame. A data voltage at a level in the positive or negative direction around a non selection scan voltage as a center is applied to data electrodes corresponding to the column of the liquid crystal display panel in accordance with the value of display data. Further, alternating operation in which the polarity of the application voltage is inverted every predetermined time is also performed.
On the other hand, as another method of driving the liquid crystal display having a passive matrix liquid crystal display panel, there is a method of selectively driving a plurality of lines described in Japanese Laid-Open Patent Publication No. 6-67628. In the method, a selective scanning voltage corresponding to an orthogonal function (for example, Walsh function) every plurality of lines is sequentially applied to scanning electrodes corresponding to a row in the liquid crystal display panel. When all of the scanning electrodes are scanned in a period, called a period of one frame, the same operation is repeated. The operation is schematically shown in FIG. 2. FIG. 2 shows a case where the number of lines simultaneously selected is eight. The data voltage corresponding to the number of coincidence of the value of the orthogonal function in the selectively scanned line and the value of display data is applied to the data electrodes corresponding to a column in the liquid crystal display panel.
In the display to which the voltage averaging method is applied, since the levels of application voltages generated by a data driver and a scan driver is shifted close to a selective scanning voltage of the scan driver at the time current alternating operation, output amplitudes are equal. The value VLCD is given as follows by using the number N of scanning electrodes and a positive constant called a bias ratio.                     VLCD        =                              (                                          N                +                1                                      )                    ·                                                    N                                            2                ⁢                                  (                                                            N                                        -                    1                                    )                                                              ·          Voff                                    (        1        )            
On the other hand, in a display to which the method of selecting and driving a plurality of lines is applied, output amplitudes Vg and Vf of the data driver and the scan driver are given by using the number m of lines simultaneously selected and the number N of scanning electrodes as follows.                     Vg        =                  2          ⁢                                    m                        ·                                                            N                                                  2                  ⁢                                      (                                                                  N                                            -                      1                                        )                                                                        ·            Voff                                              (        2        )                                Vf        =                  2          ⁢                                                    N                m                                      ·                                                            N                                                  2                  ⁢                                      (                                                                  N                                            -                      1                                        )                                                                        ·            Voff                                              (        3        )            
In the conventional liquid crystal display driving method, when a specific display pattern is displayed, display irregularity called shadowing occurs in the vertical and lateral directions. The shadowing in the lateral direction occurs since a dielectric constant of the liquid crystal cell at the time of xe2x80x9conxe2x80x9d display and that at the time of xe2x80x9coffxe2x80x9d display are different due to dielectric constant anisotropy of the liquid crystal cell.
Specifically, the dielectric constant of the liquid crystal cell when a voltage is applied (xe2x80x9conxe2x80x9d) is larger than that when a voltage is not applied (xe2x80x9coffxe2x80x9d). As the number of liquid crystal cells which are xe2x80x9conxe2x80x9d on the scanning electrodes increases, the sum of electrostatic capacity seen from the scanning electrodes increases. Consequently, the scanning electrodes on which the number of liquid crystal cells which are xe2x80x9conxe2x80x9d is large become largely weakened each time the selective scanning voltage changes and the effective value of the voltage applied to each liquid crystal cell on the scanning electrode is reduced to a value lower than a desired level. Consequently, for example, as shown in FIG. 9, when a plurality of bars having different lengths are displayed by turning on a plurality of cells on the background where the cells are xe2x80x9coffxe2x80x9d, the effective value of a voltage applied to the liquid crystal cell (difference voltage of a voltage applied to the scanning electrode and a voltage applied to the data electrode) is reduced in the row in which the bar is displayed as compared with a row in which a bar is not displayed. The longer the bar display is, the more the effective value is reduced.
With respect to shadowing in the vertical direction, waveform distortion due to change in the data voltage differs according to display patterns and the effective value of the application voltage in a certain period for determining display is different every column, so that display luminance difference (display irregularity) occurs.
It is, therefore, an object of the invention to provide a liquid crystal display in which a method of simultaneously driving a plurality of scanning electrodes on a liquid crystal display panel of a passive matrix display type is used and shadowing in the lateral direction due to dielectric constant anisotropy of the liquid crystal cell is reduced.
It is another object of the invention to provide a liquid crystal display and a method of driving a passive matrix liquid crystal, especially, a method of selectively driving a plurality of lines, in which shadowing in the vertical direction due to the difference in waveform distortion of a data voltage can be reduced.
In order to solve the problem, the invention provides a liquid crystal display having a liquid crystal display panel of a passive matrix display type having a plurality of scanning electrodes and a plurality of data electrodes, comprising: scanning electrode driving means for sequentially and simultaneously selecting (m) scanning electrodes (m is an integer of 2 or larger) corresponding to a row as a display target and applying a selective scanning voltage at a level based on a value of an orthogonal function to the scanning electrodes simultaneously selected; data electrode driving means for generating a voltage by which display data in the row can be displayed on the basis of display data of the row of the scanning electrodes simultaneously selected and the value of the orthogonal function used to determine the selective scanning voltage applied to the scanning electrodes, and applying the voltage to the plurality of data electrodes; counting means for obtaining the sum of display data which is xe2x80x9conxe2x80x9d among display data in the row of the scanning electrodes simultaneously selected every row; and selective scanning voltage correcting means for correcting the level of the selective scanning voltage applied to the scanning electrodes simultaneously selected so that the reduction in an effective value of a voltage applied to each of liquid crystal cells corresponding to the scanning electrodes on the basis of the sum of display data indicative of display xe2x80x9conxe2x80x9d in the row of the scanning electrodes and the value of the orthogonal function used to determine the selective scanning voltage applied to the scanning electrode.
The above problem is solved by a liquid crystal display comprising a liquid crystal display panel in which each of dots is formed at a crossing point of a scanning electrode and a data electrode which cross each other; a scanning electrode driving means for applying selective scanning voltages at two levels having polarities on the positive side and the negative side when a selective un-scanning voltage is used as a center in accordance with values of orthogonal function data every group of scanning electrodes obtained by setting two lines of said scanning electrodes as a set; a data electrode driving means for summing up the numbers of coincidence between a value of display data on each scanning electrode in a group of scanning electrodes to which the selective scanning voltage is applied and a value of orthogonal function data to be supplied to each of the scanning electrodes every group of scanning electrodes and for applying a data voltage according to the sum of coincidence numbers to the data electrode; and power source means for generating a voltage at a level necessary to drive the liquid crystal display panel and a power source voltage of the scanning voltage driving means and the data voltage driving means, wherein the data electrode driving means has: a latch circuit for holding the sum of coincidence numbers for one horizontal period; a correction signal generating circuit for comparing the held sum of coincidence numbers with a present sum of coincidence numbers and for generating a correction signal when the sums are different; and a voltage selection circuit for shifting the level of a data voltage by the correction signal.
That is, when the sum of the coincidence numbers in the previous horizontal period and the sum of the coincidence numbers in the present horizontal period are different, voltage change in outputs of the data driver occurs and the voltage waveform distortion occurs by the electrostatic capacity of the liquid crystal and the resistance components such as wiring. In order to compensate the distortion amount, means for shifting the voltage level is employed. By adjusting the voltage level to be corrected in accordance with the difference between the sum of the coincidence numbers in the previous horizontal period and the sum of the coincidence numbers in the present horizontal period, great effects can be obtained. In this case, the voltage is corrected by means such as amplitude adjustment, pulse width adjustment, and the like.